block: remove alloc_part_info and free_part_info

There isn't any good reason not to simply open code the allocation and
freeing of the partition_meta_info structure.  Especially as one of
the branches in alloc_part_info is entirely dead code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>